Pigs are devices that are propelled under the pressure of gas or liquid in a pipeline along the pipeline for various servicing purposes. The pig may tow other tools, for example smart pigs that analyze the condition of the inside of the pipeline, or it may stand alone and for example push debris along the pipeline and away from low spots in the pipeline. When used for towing smart pigs, it is desirable that the pig maintain a constant pre-set speed in the range for example of 4-6 mph.
There has therefore developed a need for a pig that maintains a constant speed in a pipeline, and in which the speed may be pre-set. While some have been proposed, those that the applicant is aware of have had shortcomings.
There is therefore provided in one embodiment a variable speed pig for a pipeline comprising:
a housing having at least one flow through passageway;
an annular seal disposed about the housing for sealing engagement with the pipeline and the housing; and
closure means for variably closing and opening the flow through passageway.
In one embodiment, the pig includes a plurality of tubes disposed along the housing and defining a plurality of flow through passageways; and
the closure means comprises at least one cap for closing at least one flow through passageway.
In a further embodiment, the invention includes adjusting means for adjusting the closure means in response to variations in the speed of the pig in the pipeline to maintain a desired speed of the pig.
The adjusting means may comprise:
sensor means for producing first signals indicative of the speed of the pig;
control means responsive to the first signals to produce control signals; and
drive means responsive to the control signals for variably opening and closing the closure means.
The closure means may include:
a first plate disposed in the housing, the first plate having a plurality of first openings;
a second plate disposed in the housing, the second plate having a plurality of second openings;
relation to the first plate from a position in which at least some of the first and second openings are in fluid connection to a position in which substantially none of the first and second openings are in fluid connection.
In one embodiment, the first and second openings are disposed circumferentially around the first and second plates respectively to form a plurality of flow through passageways disposed circumferentially inside the housing and defining a central cavity.
The power supply may be disposed between the circumferentially disposed flow through passageways.
In another aspect of the invention, there is provided an annular seal comprising an arcuate section extending rearward and outward from the housing; and
an angular section having a first part extending outward from the arcuate section and a second part extending rearward of the first part and having an outer surface concentric with the interior of the pipeline, the first and second parts meeting at an abrupt edge.
The arcuate section may initially extend substantially perpendicularly from the housing and includes forward and rearward substantially parallel edges, and the angular section commences rearward of the initial part of the rearward edge of the arcuate section, and the second part of the angular section terminates in a reverse angle.
In accordance with another aspect of the invention there is provided a method of controlling the speed of a pig in a pipeline comprising:
determining the actual speed of the pig;
variably controlling the size of a flow through passageway in the pig in response to determining the speed of the pig.
Preferably, variably controlling the size of the flow through passageway comprises:
pre-selecting a desired speed of the pig;
comparing the desired speed of the pig with the actual speed and decreasing the size of the flow through passageway when the actual speed is lower than the desired speed by a pre-set amount and increasing the size of the flow through passageway when the actual speed is higher than the desired speed by a pre-set amount.